It seems quite common (although not universally accepted) to open emails addressed to a large group with "Dear All," (see e.g. this question). Extending this logic, is seems reasonable to open with "Dear Both," is the group you're addressing happens to have exactly two people in it.
It goes without saying that it is not appropriate for formal letters, but I am wondering - just how inappropriate is it? Is it OK in semiformal correspondence (to one's coworkers, students, etc.)? How does it sound to the native spreakers? (i.e. Does it sound/look unnatural?)
Edit: I have tried googling, but unfortunately, this does not produce very satisfactory results. It's mostly forum threads with contrary and unsupported answers (ranging from things like "I've never seen it used" to "surely, it's been widely used even before emails").